This proposal describes a 3-year training program for the development of an academic research career, which completes the training started during a KL2 Career Development Award. The PI is a gastroenterologist who has previous experience in stable isotope tracer methods for determining metabolic processes in vivo with a focus on nutrition and obesity. She is currently expanding her scientific skills in investigation of the pathogenesis of Nonalcoholic Fatty Liver Disease (NAFLD) in relation to consumption of an increased high fructose corn syrup diet in obese persons without NAFLD compared to obese persons with NAFLD. Dr. Samuel Klein, the primary mentor for this proposal, has considerable expertise in evaluating substrate metabolism in vivo in human subjects, particularly the metabolic abnormalities associated with obesity, and has an established record of training independent scientists. To complete the investigator's scientific development, Dr. Nada Abumrad, an expert in molecular analysis, and Dr. Bruce Patterson an expert in isotope tracer methods and mathematical modeling will be secondary mentors. This proposal also includes consultation from Dr. Ken Schechtman, a biostatistician, who will provide statistical analysis support. The environment at the PI's institution provides a rich network of intellectual and investigational resources to support her career development. The proposed research focuses on a carbohydrate source that is prevalent in the United States food supply, high fructose corn syrup (HFCS), and its role in the pathogenesis of NAFLD. NAFLD affects approximately one-third of the adult population in the United States and is associated with serious cardiometabolic complications. Several lines of evidence suggest that HFCS in the diet affects triglyceride storage, lipid kinetics, and insulin sensitivity in the liver, and epidemiologic studies have correlated consumption of HFCS with incidence and severity of NAFLD and insulin resistance. However, many studies conducted in people have involved overfeeding fructose in persons with normal intrahepatic triglyceride content, which does not address the weight gain independent effect of consuming HFCS in a high-risk population. Isocaloric fructose feeding studies have shown some but not all of the metabolic changes seen in hypercaloric fructose feeding studies, and no studies have prospectively fed HFCS to obese adults with NAFLD and compared their response to HFCS to people without NAFLD. The goal of this proposal is to provide new insights into the in vivo metabolic effects of HFCS in subjects with NAFLD compared to subjects without NAFLD, clarifying the role of HFCS in the increased hepatic triglyceride storage and lipid kinetics as well as decreased multi-organ insulin sensitivity seen in patients with NAFLD. Scientific tools including magnetic resonance spectroscopy, stable isotope tracer infusions, hyperinsulinemic euglycemic clamp procedure, and molecular analysis of skeletal muscle tissue samples will assist in addressing the affect of a 4-week isocaloric diet high in HFCS in non-NAFLD obese adults compared to obese adults with NAFLD on: 1) intrahepatic triglyceride content, 2) hepatic lipid metabolism, and 3) multi-organ insulin sensitivity. The larger goal of th application and this mentored research grant is for the PI to develop the knowledge and skill sets required to become an independent investigator with R01 funding and ask relevant questions that aid in our understanding of the pathophysiology of NAFLD.